<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script type="text/javascript">
        function log(x, y = 'World') {
            console.log(x, y);
        }

        log('Hello') // Hello World
        log('Hello', 'China') // Hello China
        log('Hello', '') // Hello
    </script>
</body>

</html>